HTML partial rendered in JS view is empty

Question

I have a model Session, which has_many ExperimentSessions. In sessions/show.js.erb I render a Session's ExperimentSessions using a partial:

#/sessions/show.js.erb
<% out = '' %>
<% fields_for @session do |sess_f| %>
  <%  sess_f.fields_for :experiment_sessions, child_index: "", index:nil  do |f| %>
    <% out += j "<li class='list-group-item'>"%>
    <% out += j render partial: 'experiment_sessions/exp_sess', locals: {f:f}  %>
    <% out += j "</li>"%>
  <% end %>
<% end %>

This works well. However, I'd like to put this into its own partial view to reuse it. I have this:

#/sessions/_exp_sessions_li.html.erb
<% fields_for session do |sess_f| %>
  <%  sess_f.fields_for :experiment_sessions, child_index: "", index:nil  do |f| %>
    <%= "<li class='list-group-item'>"%>
    <%= render partial: 'experiment_sessions/exp_sess', locals: {f:f}  %>
    <%= "</li>"%>
  <% end%>
<% end %>

and include it in show.js.erb:

#/sessions/show.js.erb
<% out = j(render(partial: 'exp_sessions_li', locals: {session: @session}  )) %>

From the logs, there are no errors, and the partials get rendered. However, out is the empty string. Why does _exp_sessions_li.html.erb produce empty output?

---

Edit: clarify logs

logger.ap @session output in show.js.erb:

#<Session:0x007fc4859748d8> {
            :id => 732187210,
   :recorded_on => Fri, 20 Mar 2015 00:00:00 EDT -04:00,
         :notes => "session_j_b_ 15",
    :created_at => Fri, 20 Mar 2015 00:00:00 EDT -04:00,
    :updated_at => Fri, 20 Mar 2015 00:00:00 EDT -04:00
}

logger.ap @session.experiment_sessions shows these objects as well. session is also visible as expected in /sessions/_exp_sessions_li.html.erb.

Log indicates the partials get rendered:

Rendered experiment_sessions/_exp_sess.html.erb (0.7ms)
Rendered experiment_sessions/_exp_sess.html.erb (0.6ms)
Rendered sessions/_exp_sessions_li.html.erb (4.9ms)

---

Edit: drill down on partial

Replacing the contents of _exp_sessions_li.html.erb with a string will render that string. Furthermore, if I edit this file to be

aa
<% fields_for session do |sess_f| %>
  bb
  <% sess_f.fields_for :experiment_sessions, child_index: "", index:nil  do |f| %>
    cc
    <%= "<li class='list-group-item'>"%>
    <%= render partial: 'experiment_sessions/exp_sess', locals: {f:f}  %>
[...]

will only show 'aa' in out. So fields_for seems to eat the output. Note that the logs still say exp_sess gets rendered.

Now, if I write <%= fields_for session do |sess_f| %>, output also contains 'bb'. And adding <%= sess_f.fields_for ... also the rest. The raw HTML parts (<li ..) are escaped, however.

Answer 1

OK, my mistake was to not return the output of the fields_for blocks.

The reason my original code works is that I add the output of every line to my output variable (including output of j calls, which were the only reason the <li ... lines were sent through ruby in the first place).

Simply writing <%= fields_for… solves my issue.